1. Field of the Invention
The present invention relates generally to fabric, and more particularly to fabric which conducts electricity. Such a fabric may find application in the manufacture of antistatic clothes, static charge removal and radio-interference prevention shields of electrical and electronic devices, pressure sensors etc. The invention also relates to a method of manufacturing the aforementioned electroconductive fabric.
2. Description of the Prior Art
In recent years, electroconductive fabric finds ever growing practical application. One example of such applications is so called textile-based electronics, called “electrotextiles.” Most of the ongoing research in electrotextiles is driven by the motivation of creating multifunctional fiber assemblies that can sense, actuate, communicate, etc. Wired interconnections of different devices attached to the conducting elements of these circuits are made by arranging and weaving conductive threads so that they follow desired electrical circuit designs.
Another application for electroconductive fabrics is the removal of electrostatic charge from the body of the wearer. The use of such clothes is especially important for workers who operate in clean rooms, e.g., on assembly lines of printed circuit boards, or the like. This is because the electroconductive clothes prevent accumulation of an electric charge and thus the possibility of undesired discharge, e.g., a gas discharge in the operation environment of a clean room. Under conditions of production of electronic devices that are very sensitive to electromagnetic interference, such discharges may destroy an intricate circuit of electronic device components at the production stage.
Electric discharges caused by the accumulation of static electricity accumulated on clothes may be a reason for explosion in hazardous environments with vapors of highly volatile liquids, e.g., gasoline, alcohols, explosives, TNT etc. Cases are known where a small spark caused by the discharge of static electricity from clothes caused the explosion of gasoline vapors which had accumulated in the ambient air. Similarly the combustion of liquid gasoline at the dispensing end of the gas pump has occurred.
One nuisance of static electricity is that many people get unexpected shocks, simply from touching some metal object after walking across the room. One of the biggest complaints that people have about static electricity is that it causes sparks or gives them mild shocks when they touch objects or people. There are also some situations where excess of static electricity can damage equipment or even pose a danger. The reason for the phenomenon is that when certain materials rub together, they build up static electricity. Items that commonly rub together to cause static electricity are clothes rubbing on human skin, furniture and car seats, and soles of shoes rubbing against a rug or floor, etc.
Another example of an application of electroconductive fabric is custom seats. In order to combat static electricity buildup in standard wool seat covers, Oregon Aero Co. developed the Anti-Static Inner Upholstery which draws off the static charge and directs it to the seat frame. This is especially important for aircraft seats that are packed with extensive electronic equipment and need anti-static inner upholstery.
There may be many other application examples for antistatic fabric: the use of electroconductive fabrics for heating sportwear, the lining of the casings of electronic devices for shielding against electromagnetic radiation to prevent electromagnetic interference with radio receivers, TV sets, telephones, etc., cable shielding, and military uses for special devices equipped with protective electroconductive fabric coatings that provide a predetermined electromagnetic impedance thereby screening against radio location.
Chomerics, a division of Parker Hannifin, has unveiled a line of Soft-Shield 5000 EMI gaskets designed to meet shielding and mechanical performance requirements of commercial electronic enclosures used in cellular communications. The gaskets consist of an electrically conductive fabric jacket over soft urethane foam.
Woven into The Hub snowboard jacket are electrically conductive fabric tracks that connect a chip module to a fabric keyboard and built-in speakers in the helmet. The chip module contains an MP3 player and Bluetooth capabilities from which the snowboarder can control a mobile phone. When the phone feature is used, the stereo system acts as the headset. The microphone is integrated into the collar of the jacket.
The smart-fabric and interactive-textile market has growth potential and according to a Boston-based research firm, worldwide sales could reach approximately $1 billion for “intelligent textile materials” by 2007.
It should be noted that the use of the electroconductive fabric should not impair the basic function of the object. For example, in the case of cloth or seats the product should present an attractive appearance, have wearability, and in sport-wear be lightweight and durable, etc.
The fabrics used for EC purposes may be woven, non-woven, synthetic and natural, etc. There are many methods of manufacturing. The fabric can be woven entirely from electroconductive threads, or electroconductive threads can be interweaved with conventional threads. In addition, the electroconductive fabric may have different patterns of weaving, etc.
For example, electroconductive fabrics can be manufactured by mixing or blending a conductive powder with a polymer melt prior to the extrusion of the fibers from which the fabric is made. Such powders may include, for instance, carbon black, silver particles or even silver- or gold-coated particles. When conductive fabrics are made in this fashion, however, the amount of powder or filler required may be relatively high in order to achieve any reasonable conductivity and this high level of filler may adversely affect the properties of the resultant fibers. It is theorized that the high level of filler is necessitated because the filler particles must actually touch one another in order to obtain the desired conductivity characteristics for the resultant fabrics.
Such products have some significant disadvantages. For instance, the mixing of a relatively high concentration of particles into the polymer melt prior to extrusion of the fibers may result in undesired alteration of the physical properties of the fibers and the resultant textile materials. Also, it is difficult to spin fibers which are highly loaded with conductive particles. Dark colors which result from this process may also be unwanted.
Antistatic fabrics may be made by incorporating conductive carbon fibers, or carbon-filled nylon or polyester fibers in woven or knit fabrics. Alternatively, conductive fabrics may be made by blending stainless steel fibers into spun yarns used to make such fabrics. While effective for some applications, these “black stripe” fabrics and stainless steel containing fabrics are expensive and of only limited use. Also known are metal-coated fabrics such as nickel-coated, copper-coated and noble metal-coated fabrics. However the process to make such fabrics is quite complicated and involves expensive catalysts such as palladium or platinum, making such fabrics impractical for many applications. There are also carbon black-inpregnated fibers which are widely used in the clean room industry. These black fibers are used in combination with white insulating fibers to produce the desired fabric resistance and lighter colors. However, this conductive grid might lead to a hot and cold spot phenomenon whereby this fabric could still accumulate static charges.
Polypyrrole can be produced by either an electrochemical process where pyrrole is oxidized on an anode to a desired polymer film configuration or, alternatively, pyrrole may be oxidized chemically to form polypyrrole by using ferric chloride or other oxidizing agents. While conductive films may be obtained by means of these methods, the films themselves are insoluble in either organic or inorganic solvents and, therefore, they cannot be reformed or processed into desirable shapes after they have been prepared.
The problems inherent in antistatic fabrics made from the use of chemical oxidative precipitation of conductive polymers on textile substrates were overcome by the processes disclosed in U.S. Pat. No. 4,803,096 issued on Feb. 7, 1989 to H. Kuhn, et al. The authors of the aforementioned patent have found that textile substrates can be made more uniformly electrically conductive, with adherent polymer coatings, and with reduced waste of reactants, by bringing the textile substrate into contact with an aqueous solution of a pyrrole or aniline compound and an oxidizing agent and a doping agent or counter ion, while constantly agitating the solution; thereby depositing onto the surface of the individual fibers of the textile substrate the forming polymer or prepolymer of the pyrrole or aniline monomer. By such means a uniform and coherent covering of an ordered, conductive film of the polymerized pyrrole or aniline compound is generated on the surface of the substrate fibers.
While the process previously described in U.S. Pat. No. 4,803,096 provides significant improvements over the prior art techniques, nevertheless, in practice it is often difficult to provide the precise process controls required to appropriately adjust the rates of polymer formation and adsorption, especially within appropriate boundaries for a commercial process. The use of low reaction temperatures, e.g. down to about 0° C. or below, for slowing the reaction rate is often inconvenient and adds additional expense to the overall process by virtue of increased energy costs and increased production time per unit of product.
It was pointed out in the prior application that one controlling factor in assuring that the forming pre-polymer species forms at an appropriate rate to be taken up on the textile material without forming polymer in solution is the availability or concentration of the oxidant in the aqueous solution.
The authors of the above invention made an attempt to control the availability and concentration of the iron salt oxidant, particularly FeCl3 in the aqueous solution as a means of controlling the rate of oxidative polymerization of the pyrrole monomer. However, the addition of conventional complexing agents for ferric Fe+3 ion, such as ethylene diamine tetraacetic acid (EDTA) and potassium thiocyanate (KSCN), completely stopped the polymerization of pyrrole, presumably by virtue of forming irreversible or strong complexes with Fe+3 and preventing oxidation of the pyrrole monomer to the reactive species.
However, further research has led to the discovery that there is a class of compounds which are presumably capable of forming weak complexes with Fe+3 and that when these complexing agents are included in the aqueous solution with the pyrrole monomer they effectively, controllably release the ferric ions and allow the polymerization to proceed at a rate such that the forming prepolymer species is deposited onto the surface of the fibers of the textile material as quickly as it is formed. As a result of this controlled release of ferric ions the conductive polymer film can be formed on the textile material at room temperature with uniform and coherent properties normally obtainable otherwise only at substantially lower temperatures (e.g. about 0° C.) in the absence of the complexing agent.
According to the above invention, it was found that the addition to the aqueous solution of pyrrole monomer, ferric oxidant, and optional dopant or counter ion, of certain complexing agents for the ferric oxidant provides a more effective means for controlling the rate of polymer formation.
Another invention aimed at the manufacture of conductive fabrics based on the use of a pyrrole compound is disclosed in U.S. Pat. No. 4,877,646 issued on Oct. 31, 1989 to Hans H. Kuhn, et al. Fabrics are made electrically conductive by exposing the fabric fiber under agitation conditions to an aqueous solution of a pyrrole compound, an oxidizing agent and a doping agent or counter ion and thereby depositing onto the surface of individual fibers of the fabric a prepolymer of the pyrrole compound so as to uniformly and coherently cover the fibers with a conductive film of the polymerized pyrrole compound and wherein, furthermore, the oxidizing agent is a ferric salt and the aqueous solution further contains a weak complexing agent for ferric ions to effectively control the reaction rate such that the prepolymer is uniformly and coherently adsorbed onto the surface of the textile material, thereby providing improved films of electrically conductive polymerized compound on the textile material.
U.S. Pat. No. 4,975,317 issued on Dec. 4, 1990 to Kuhn; Hans H. is aimed at further improvement of electrically conductive textile materials and method of their manufacture. According to the above invention, fabrics are made electrically conductive by contacting the fabric under agitation conditions with an aqueous solution of a pyrrole or aniline compound, and an oxidizing agent and a doping agent or counter ion; and then epitaxially depositing onto the surface of the individual fibers of said fabric the in status nascendi [in the nascent state] forming polymer of the pyrrole or aniline compound so as to uniformly and coherently cover the fibers with an ordered conductive film of the polymerized pyrrole or aniline compound. Individual fibers and yarns can be similarly treated and then formed into fabrics.
The next US Patents of Kuhn; Hans H., i.e., U.S. Pat. No. 4,981,718 issued on Jan. 1, 1991 and U.S. Pat. No. 5,030,508 issued on Jul. 9, 1991 relate generally to a method for making electrically conductive textile materials by contacting the fiber under agitation conditions with an aqueous solution of an aniline compound, oxidizing agent and a doping agent or counter ion and then depositing onto the surface of individual fibers of the fabric a prepolymer of the aniline compound so as to uniformly and coherently cover the fibers with a conductive film of the polymerized aniline compound and wherein, furthermore, the oxidizing agent is a vanadyl compound whereby the reaction rate is controlled such that the prepolymer is uniformly and coherently absorbed onto the surface of the textile material, thereby providing improved films of electrically conductive polymerized compound on the textile material.
The second basic improvement in the inventions relating to manufacture of electroconductive fabric with the use of pyrrole is disclosed in U.S. Pat. No. 5,108,829 issued on Apr. 28, 1992 to H. Kuhn. The process described in this patent is characterized by introduction in the fabric treatment solution of anthraquinone-2-sulfonic acid as a dopant. According to one embodiment of the aforementioned invention, a method is provided for imparting electrical conductivity to textile materials by exposing the textile material to an aqueous solution of an oxidatively polymerizable pyrrole compound and an oxidizing agent capable of oxidizing said pyrrole compound to a polymer. This reaction is carried out in the presence of an anthraquinone-2-sulfonic acid or sulfonate as a counter ion or doping agent to impart electrical conductivity to the polymer, and under conditions at which the pyrrole compound and the oxidizing agent react with each other to form a conductive polymer coating on the textile material. An advantage of this invention is that a large excess of dopant is not required to achieve high conductivity in the conductive textile material. Another advantage is that in addition to high conductivity, the textile material demonstrates superior stability.
In some applications, it is desirable that a material incorporating a conductive polymer exhibit anisotropic properties, i.e. non-uniform conductivity, such as a gradient of decreasing conductivity in a particular direction. The invention aimed at the solution of this problem is disclosed in U.S. Pat. No. 5,162,135 issued on Nov. 10, 1992 to R. Gregory et al. The invention relates to a conductive polymeric material such as a textile fabric having a conductive polymer film that may be treated with a solution containing a chemical reducing agent to reduce its conductivity. By selectively reducing portions of the conductive polymer in varying degrees, a gradient of conductivity may be produced in the material. After the conductive polymer has been reduced to a target level, the reducing solution may be removed with a hot water rinse.
An example of other inventions related to manufacturing electroconductive fabric is U.S. Pat. No. 5,102,727 issued on Apr. 7, 1992 to Pittman; Edgar H., et al. This invention provides an electrically conductive textile fabric having a conductivity gradient created by varying the relative concentration of high and low conductivity yarns during construction of the fabric. In the case of woven and knitted fabrics, the relative number of high and low conductivity yarns per inch may be varied in the warp or weft direction or both.
U.S. Pat. No. 6,117,554 issued on Sep. 12, 2000 to S. Shalaby, et al. describes modulated molecularly bonded inherently conductive polymers on substrates with conjugated multiple lamellae and shaped articles thereof. Organic inherently conductive polymers, such as those based on polyaniline, polypyrrole and polythiophene, are sequentially formed in-situ onto polymeric surfaces that are chemically functionalized to molecularly bond the conductive polymers to the substrates. The polymeric substrate is preferably a preshaped or preformed thermoplastic film, fabric, or tube, although other forms of thermoplastic, and thermoset polymers can be used as the substrates for functionalization using, most preferably, phosphonylation-based processes followed by exposure to an oxidatively polymerizable compound capable of forming an electrically conductive polymer. It has been found that the degree of electrical conductivity may be modulated by bonding further electrically conductive layers to the article. That is, each underlying conductive layer is functionalized prior to bonding of a subsequent conductive layer thereto until the degree of conductivity is achieved. In an alternative embodiment, metals such as gold or platinum may be bonded to one of the functionalized surfaces.
Broadly, the method of the above invention is directed to a surface functionalizing step which renders the outer surface of the polymeric article reactive by providing acid-forming functional groups with each group having a multivalent central atom followed by a polymerization step whereby a precursor monomer of a conductive polymer is polymerized directly onto the reactive surface. In addition to providing for molecular bonding of the conductive polymer to the article's surface, the functional groups act, as least in part, as both a doping agent and an oxidizing agent to aid in polymerization. A second conductive polymer layer is similarly formed onto the initial conductive polymer layer. That is, the initial conductive polymer layer is then subjected to the above-mentioned surface functionalizing step whereby acid-forming functional groups are bonded to that initial layer; a precursor monomer of a conductive polymer is then polymerized directly onto that reactive surface. The second conductive polymer layer may be identical to or different from the initial conductive polymer layer. Subsequent conductive polymer layers are formed onto underlying layers in the same way.
The Kuhn process (in-situ polymerization and deposition of ICPs) is working well for polar surfaces of such substances as nylon, polyester, Kevlar, glass, cotton etc, but the adhesion of ICPs to PE and PP is not good. Therefore, U.S. Pat. No. 6,117,554 is mostly targeting the problem of adhesion of inherently conductive polymers (ICPs) to the surface of polyolefines (polyethylene PE, polypropylene PP) and teflons. U.S. Pat. No. 6,117,554 describes a modification of the hydrophobic polymer surface to introduce acidic groups. In fact, this is essentially the same process as described above in the Kuhn and Kuhn et al. patents with pre-treatment of the polymer surface.
Thus, it can be concluded that the methods and textile structure described in U.S. Pat. No. 6,117,554 will entail the disadvantages of the Kuhn technique since it uses essentially the same ICP, though with some improvement of chemical adhesion to PP and PE.
An attempt has been made to improve stability against heat, moisture, and laundering by hindering the extraction of dopants. Such an attempt is disclosed in by S. A. Asharaf, et al. in the report at the Avantex Symposium, Frankfurt, June 2005. The proposed method is based on the following two concepts: 1) Diffusing ICPs into the fabric fibers by the prolonged impregnation method and; 2) Preadsorption of large dopant molecules (templates or polymeric acids) onto fabric for subsequent use in situ by the Kuhn method. Although the S. A. Asharaf method can be considered as a next step towards improved thermal and environmental stability of the electroconductive fabric as compared to the Kuhn's technique, S. A. Asharaf's is inferior to Kuhn's technique with regard to electroconductivity.
U.S. Pat. No. 6,316,084 issued on 2001 to Richard Claus et al. relates to transparent abrasion-resistant coatings, magnetic coatings, electrically and thermally conductive coatings, and UV absorbing coatings on solid substrates. Abrasion and scratch protective coatings magnetic coatings, electrically and thermally conducting coatings, and UV absorbing coatings are provided by electrostatic self-assembly (ESA) of one layer of an organic or polymer molecule and one layer of inorganic clusters in a layer by layer fashion at room temperature. A combination of inorganic clusters having a particle size of preferably less than 30 nm and flexible organic molecules allows fabrication of films tens to hundreds of micrometers thick, with large pores and excellent stress relaxation.
Although the method disclosed in that patent relates mainly to application of wear-resistant coatings, one of the objects also includes imparting to the substrate material electroconductive properties by application of coatings with the proposed method.
Another US Patent issued to Richard Claus et al. in 2002 is U.S. Pat. No. 6,447,887 describes electrostrictive and piezoelectric thin film assemblies and method of fabrication therefor. The electrostatic self-assembly method of fabricating electrostrictive and piezoelectric thin film assemblies not only provides a thinner film than is attainable by conventional methods, but provides excellent molecular-level uniformity and precise structural control, and thus large, effective piezoelectric coefficients. The method produces a thin film assembly including (a) a substrate, and (b) a film having one or a plurality of layers disposed upon the substrate, wherein at least one of the layers includes a dipolar material, and this layer of dipolar material has a uniform thickness of at most 500 nm.
An invention that is worth mentioning in connection with the subject of the present invention is one disclosed in U.S. Pat. No. 5,536,573 issued on 1996 to Michael Rubner. The invention relates to a molecular self-assembly of electrically conductive polymers. A thin-film heterostructured bilayer is formed on a substrate by a molecular self-assembly process based on the alternating deposition of a p-type doped electrically conductive polycationic polymer and a conjugated or nonconjugated polyanion or water soluble, non-ionic polymer. In this process, monolayers of electrically conductive polymers are spontaneously adsorbed onto a substrate from dilute solutions and subsequently built-up into multilayer thin films by alternating deposition with a soluble polyanion or water soluble, non-ionic polymer. In contrast to a deposition process involving the alternate self-assembly of polycations and polyanions, this process is driven by non-covalent bonded attractions (for example, ionic and hydrogen bonds) developed between a p-type doped conducting polymer and a polymer capable of forming strong secondary bonds. The net positive charge of the conducting polymer can be systematically adjusted by simply varying its doping level. Thus, with suitable choice of doping agent, doping level and solvent, it is possible to manipulate a wide variety of conducting polymers into uniform multilayer thin films with layer thicknesses ranging from a single monolayer to multiple layers. This method leads to conductive coatings that are still vulnerable to heat, moisture, ultraviolet radiation and washing.
In spite of a great variety of methods for manufacturing and treating electroconductive textiles, there is still room for the improvement. For example, a common disadvantage which remains for the conventional electroconductive textile is that with the lapse of time electrical characteristics are impaired, at least in some applications. Another disadvantage of the known electroconductive fabrics is that they are insufficiently stable to environmental conditions, such as humidity and temperature. Electroconductive textile materials, e.g., those that are produced by the Kuhn method, are not sufficiently resistive to laundering. This is because the dopant used in the above method is leachable, i.e., has water-soluble molecules.